In fact, vitamin D plays a pivotal role in the immune system. The explanation likely comes from the fact that vitamin D in cod liver oil does not exist in isolation — it comes with a high dose of vitamin A.

Vitamin A and vitamin D compete for each other’s function. For example, even the vitamin A in a single serving of liver can impair vitamin D’s rapid intestinal calcium response.

Unfortunately, Americans tend to consume multivitamins or cod liver oil that contain disproportionately small amounts of vitamin D, but detrimental quantities of vitamin A. One manufacturer sells cod liver oil containing only 3 to 60 IU of vitamin D, but between 3,000 and 6,000 IU of vitamin A.

A separate study by Daniel Hayes, Ph.D., of the New York City Department of Health and Mental Hygiene also suggests that a form of vitamin D could be one of your body’s main protections against damage from low levels of radiation. Hayes explains that calcitriol, the active form of vitamin D, may protect us from background radiation and could be used as a safe protective agent before or after a low-level nuclear incident.

He points out that calcitriol is involved in cell cycle regulation and control of proliferation, cellular differentiation and communication between cells, as well as programmed cell death (apoptosis and autophagy) and antiangiogenesis.

Calcitriol is the form of vitamin D that activates your body’s Vitamin D Receptor (VDR), which allows gene transcription to take place and the activation of the innate immune response.

It is possible that several of the transcriptions by the VDR will help transcribe proteins that protect the body against radiation.